fix(ext/node): run register() hooks in worker thread, add --experimental-loader flag (#33906)

Two related features that together bring Node.js module loader hooks
much closer to compatibility:

### 1. Worker-thread architecture for `module.register()` 

Rearchitects `module.register()` to run async hooks in a dedicated Web
Worker thread, matching Node.js's "hooks thread" design. Previously,
hook modules were loaded on the main thread, causing deadlocks when the
hook module's own imports went through the hook infrastructure.

**How it works:**
- On first `register()` call, a Web Worker is spawned with inline
bootstrap code
- The hook module is loaded inside the worker (separate event loop, no
deadlock)
- Resolve/load requests are forwarded from the main thread to the worker
via `postMessage`
- Sync hooks from `registerHooks()` still run on the main thread (per
Node.js spec, sync before async)

### 2. `--experimental-loader` CLI flag

Adds `--experimental-loader` (with `--loader` alias) as a hidden CLI
flag for Node.js compatibility. Before the main module runs, each loader
module is eagerly imported and its resolve/load hooks are registered.

### 3. Fix sync resolve panic (#33901)

Adds a `resolve_cache` to `LoaderHookRegistry` that caches resolve
results from the async load phase. During V8's synchronous module
instantiation callback, the cache is checked first, returning a sync
result instead of panicking on an async response.

Closes #23201
Fixes #33901